1. Field of the Invention
The present invention relates to a system for playing back information recorded on a recording disc, and more specifically to a system in which the position of an information reading point of a pickup is adjusted in a radial direction of the disc by a servo control system and the speed of rotation of the recording disc is controlled by a spindle servo control system.
2. Description of Background Information
In systems for playing back information signal recorded on a recording disc such as a digital audio disc player system or a video disc player system, the position of an information reading point of the pickup (focal point of the reading light beam in the case of optical playback systems) is generally controlled by a tracking servo system and a carriage servo system which controls the position of a carriage carrying the pickup in a radial direction of the recording disc. In addition, the rotational speed of the disc is controlled by the operation of a spindle servo system which is responsive to a synchronizing signal such as a frame synchronizing signal which is regularly produced when the information is normally read out from the recording disc.
In conventional systems of this type, the absence of the frame synchronization signal (which is to be produced regularly) is detected so that the servo control operation of the spindle servo system is stopped and the resumption of the servo control operation is made when the regular generation of the frame synchronization signal is assured. However, those systems were not provided with means for providing a mutual relation between the control of the spindle servo system and the control of the tracking servo system and the carriage servo system.
Therefore, it was often the case that the regular reproduction of the synchronization signal is continuously disturbed when a tracking deviation, i.e., the deviation of the information reading point of the pickup from the recording track, occurs due to an external disturbance such as a shock or vibration is applied to the system.